Humoral immunity is an essential component of the immune response to flavivirus infection. Primary infection generates a robust neutralizing antibody response that mediates viral control and protection. It is becoming increasingly apparent that secondary infection with a closely related flavivirus strain can result in immunological cross-reactivity; however, the consequences to infection outcome are hotly debated and controversial. Zika virus (ZIKV) is a mosquito-borne flavivirus, which has a high degree of sequence and structural homology to Dengue virus (DENV), and is responsible for continuing epidemics of fetal congenital malformations within the Americas since its introduction to Brazil in 2015. Prior flavivirus exposure has been strongly associated with generation of cross-reactive antibodies that bind to and/or neutralize ZIKV. A unique aspect of ZIKV pathogenesis is the ability of the virus to seed infection within the placenta, however, the mechanisms of transplacental ZIKV infection are not well understood. The overall goal of this proposal is to understand how cross-reactive antibodies facilitate ZIKV transcytosis and seed infection of the placenta. The placenta is composed of anchoring chorionic villi, which penetrate the uterine wall, as well as floating chorionic villi that are bathed in maternal blood pooling in the intervillous space. Recent epidemiological observations found that between 20-50% of pregnant women with possible ZIKV exposure had detectable ZIKV RNA in the placenta. Another report found that ZIKV can persist in the placenta for over 200 days post mother onset of Zika symptoms. We discovered that Hofbauer cells, fetally- derived placental macrophages located within the villus stroma, are permissive for ZIKV infection. To identify a potential mechanism by which ZIKV gains access to the villous stroma, we recently evaluated the impact of cross-reactive dengue antibodies in mediating transplacental infection. Using an ex vivo placental explant model, we observed profound enhancement of ZIKV infection of human mid-gestation floating chorionic villi with ZIKV immune complexes generated using either DENV or ZIKV cross-reactive convalescent serum or monoclonal antibodies. Similar to histological analysis of placenta from infected pregnant mothers, ZIKV replicated exclusively within Hofbauer cells. Based on these observations, we hypothesize that the Fab fragment (specificity for ZIKV) and the Fc domain (affinity for FcRn and Fc?R) of IgG impacts antibody-mediated ZIKV transplacental infection. Moreover, we believe that gestational age of the placenta dynamically influences the efficiency of ZIKV transcytosis and placental infection. Moreover, we believe that gestational age of the placenta dynamically impacts ZIKV transcytosis and placental infection. In this proposal, we seek to address the following outstanding questions: 1) How does IgG antibody specificity, affinity and Fc/FcRn interactions impact ZIKV transplacental infection? and 2) How does placental gestational age impacts antibody-mediated infection of Hofbauer cells? Our studies will likely reveal therapeutic targets and provide insights for development a vaccine to protect against ZIKV infection.